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The ultimate objective of this work is to leverage modern computational tools to provide a unique and contemporary approach to pulse sieve-plate extraction column (PSEC) design and optimisation. Particular attention is given to providing novel analysis on: the functionality of operation, methods of performance analysis, determination of flooding, and development of simulation approaches that faithfully represent PSEC hydrodynamic behaviour. A detailed assessment is undertaken of the dispersive mixing and turbulence characterisation of an industrially representative PSEC. This is achieved with computational fluid dynamics (CFD) running turbulence resolving large eddy simulation (LES), coupled with the volume of fluid (VOF) multiphase approach. Found was the dependency of PSEC functionality on turbulence production, and not on the viscous plate-induced stresses, generated therein. Consequently, the standard round-hole sieve-plate design is found to perform poorly at producing and distributing the types of flow and turbulence beneficial to droplet size reduction. This milestone discovery marks the first explicit contribution to knowledge of PSEC operation in decades. Subsequently, a number of typical unsteady Reynolds averaged Navier-Stokes (URANS) turbulence modelling methods were compared against the benchmark LES. The URANS models, highly representative of the available PSEC CFD literature, were not able to produce agreeable solutions in the important hydrodynamic characteristics of the flows. Therefore the standard has been set for turbulence characterisation in PSEC simulation with LES. The appropriate LES VOF method was carried forward to a campaign of 25 unique case runs that resulted in synergistically rich data set. Novel means of flooding identification was developed and tested. From this a number statistical analysis methodologies were employed to develop tools which successfully resolve the operational envelope and diagnose the likelihood of flooding during operation based on easily measurable variables. Lastly, a state-of-the-art two-fluid hybrid VOF/Eulerian-Eulerian multiphase CFD model, with population balance, was implemented and interrogated. The model was successful in capturing all scales of the multiphase behaviour to further improve the faithful description of the complete fluid interactions. The population balance produced predictions for the droplet size distributions inline with available examples from literature and therefore provides exciting opportunities for accurate mass transfer predictions in pulse column simulations.

Upon removal from a reactor spent nuclear fuel is placed into water storage for a period of time since water provides both a decay heat sink and radiation shielding. In the UK this fuel has traditionally been reprocessed however the decision has been made that this will cease in 2018 in favour of an open fuel cycle and direct disposal to a degological disposal facility, although this is not expected to be available until 2075. Current experience with pond storage has found that fuel can be safely stored in caustic dosed ponds for at least 25 years and therefore the current plan is to extend water storage until around 2040, however how the fuel will be stored for the remaining 35 years is yet to be decided. One option that is being considered is the use of dry storage which is being used successfully in the USA for the storage of fuel from Light Water Reactors. One of the most important factors when utilising dry stores is an effective drying step and this PhD is aiming to look at this area in particular. Commercial drying methods have been developed for use with Zircaloy and aluminium clad fuels however this PhD intends to develop a method that can be used to dry Stainless Steel clad fuels such as those used in the UK's Advanced Gas Reactors (AGR). Since intact AGR fuel has yet to be examined following storage the first part if this thesis looked at preparing samples for later macro scale testing. Samples were treated to induce defects known to be of concern and analysed with the conclusion being that bound water is of no concern for drying operations and can be disregarded when preparing samples. Since AGR is known to have failed by stress corrosion cracking water trapped within microcracked pins is a likely issue. A section of tube was prepared by compression and boiling in magnesium chloride which micro CT imaging found to have images of a similar size to known failures. This tube was prepared into a test piece for later testing. The second part of the thesis looks at macro scale testing using the test piece described above and a similar test piece with a 300 um pinhole. A multipurpose drying rig capable of vacuum drying and flowed gas drying was constructed. In the first phase of drying tests vacuum drying and flowed gas drying were compared using a test piece with a 300 um pinhole and the impact of different cover gas was assessed. vacuum drying was found to be significantly more effective than flowed gas drying with the type of cover gas having little impact. Most importantly this work guided further improvements to the rig. The second phase of this work carried out more detailed drying tests on vacuum drying and flowed gas drying and utilised the pinholed and cracked test piece. The greatest influencing factor on the drying rate (the rate at which water could be removed from inside the test piece) was found to be the water level inside the test piece with broadly similar rates being found in all conditions and with both test piece when the test piece was full. As the water level within the test piece dropped, vacuum drying quickly became more effective. This was not helped by considering energy usage. There was however a feeling that flowed gas drying was limited due to the pipework size. In the final phase of testing the ability to detect the point which all water was removed (the end point) from the test piece using online instrumentation was assessed. Currently a vacuum rebound test is used which is both time consuming and if flowed gas drying is being used requires additional plant equipment. It was found that dew point measurements appeared to give a relatively clear indication of the end point when for both vacuum drying and flowed gas drying. For vacuum drying only the measurement of mass flow and pressure also gave a reasonable indication of end point.

Calcium uranyl(VI) oxyhydrates and uranates are structurally related U(VI)-phases featuring uranium oxo-polyhedral sheets, with calcium ions occupying the interlayer. Both coordination environments appear throughout the nuclear fuel-cycle as alteration products, colloids, and sorption complexes. However, concerted studies spanning the aqueous precipitation mechanisms of uranyl(VI) oxyhydrates, their solid-state transformations, and structural relationships with uranates, have hitherto remained largely unexplored. A series of calcium-based uranyl(VI) oxyhydrates were precipitated via alkalisation of aqueous precursor solutions in titration and batch reactions. The bulk stoichiometric ratio of calcium to uranium (Ca/U) of precipitates was varied by modifying precursor stoichiometry, reaction temperature, or extraction pH. The rate of precipitation and its dependency on temperature was quantified in-situ using a quartz crystal microbalance. Novel insight was revealed on the mechanisms influencing nucleation and growth, by determining associated kinetic barriers as a function of precursor-Ca/U. Remarkably, as the bulk precipitate Ca/U increased from ~⅛ to unity, there was a transition from crystalline Becquerelite to primary or secondary amorphous phases, with uranate-like coordination environments. Formation of the latter was driven by solution alkalinity, and comprises a poorly-ordered matrix with occlusions of Ca2+-rich nano-clusters. A congruency limit lies Ca/U of ~1.5 Ca/U, whereupon discrete Portlandite crystallises. Solid-state transformation of all Ca2+-U(VI)-phases studied involved dehydration, dehydroxylation-decarbonation, and desorption processes. Associated kinetic barriers were catalysed by higher Ca2+-contents, and was reflected by reaction enthalpies for dehydration and desorption. Crystalline Becquerelite (~⅛ Ca/U) underwent amorphisation-crystallisation via partial egress of interlayer calcium, followed by reduction of β-UO3 to form a novel intercalation compound Ca0.18.α-U3O8. The endmember uranates Ca3U11O36, CaU2O7, Ca2U3O11, and CaUO4 crystallised from amorphous precursors with higher bulk Ca/U (~⅓, ~½, ~⅔, ~1), where Ca3U11O36 is a novel compound that is isostructural to (Pb/Sr)3U11O36. Nucleation and growth became predominant in the presence of Ca2+-rich occlusions. A higher Ca2+-loading facilitated the progressive ingress of interlayer-Ca2+, inducing a concerted axial compression in uranyl(VI) oxo-polyhedra towards the uranate-like coordination environment.